The immune system is the complex combination of organs, cells and chemicals that the body uses to defend itself against invasion by bacteria, viruses, fungi, parasites, toxins, and foreign bodies. The immune system carries out this protective action by distinguishing between "self" and "nonself." In other words, the immune system recognizes the cells and chemicals that are supposed to be part of us, and recognizes cells and chemicals that are not. Occasionally this recognition breaks down, and the body's immune system attacks some part of the host's own body, in a process called autoimmune disease.

Despite the protection the immune system offers, occasionally bacteria and viruses do infect us; or more dramatically, we are pierced by a splinter or some other foreign object. The immune system's response is a significant component of the symptoms we feel. Fever, the production of phlegm, pus, redness, swelling, pain and tenderness at the site of an infection, all result in large part from the actions of our immune system to respond to this invasion.

Still, despite its occasional excesses, a successful immune response makes the difference in whether we live or die as a result of an infection. For example, the poor health that can be experienced by persons with AIDS is a demonstration of what life can be like without a functional immune system, and the tendency of older adults to be at greater risk with a bout of flu or bacterial pneumonia is also a reminder that the loss of immune protection with aging can have serious health consequences.